The present invention is directed, in general, to CDMA wireless networks and, more specifically, to a system for reallocating a traffic channel in a soft handoff state as an overhead control channel or as a traffic channel for another mobile station.
Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered not only by dedicated cellular service providers, but also by the regional Bell companies, such as U.S. West, Bell Atlantic and Southwestern Bell, and the national long distance companies, such as ATandT and Sprint. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population.
Wireless subscribers use a wide variety of wireless devices, including cellular phones, personal communication services (PCS) devices, and wireless modem-equipped personal computer (PCs), among others. The large number of subscribers and the many applications for wireless communications have created a heavy subscriber demand for RF bandwidth. To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
Although TDMA-based wireless networks were developed on a large scale first, CDMA-based wireless networks are now also widely used. CDMA systems divide the RF spectrum into a number of wideband digital radio signals. Each digital radio signal carries several different coded xe2x80x9cchannelsxe2x80x9d. Each coded channel is distinguished by a unique pseudo-random noise (PN) code used by the mobile station and/or the base station. In a CDMA receiver, the coded channels are decoded by a signal correlator that matches PN sequences.
Some coded channels are used as data traffic channels to transport subscriber voice and/or data signals, while other coded channels are used as control, or xe2x80x9coverhead,xe2x80x9d channels, including Pilot, Synchronization, Paging and Access channels. In some systems, one or more of the Pilot, Synchronization, Paging and Access channels may be combined into a single channel.
When a mobile station accesses a base station in a CDMA wireless network via the overhead channels (each of which has a unique PN code), the network assigns the mobile station to a data traffic channel (which has a different PN code than the overhead channels) on which the mobile station exchanges voice or data traffic with another party (including another mobile station), a data terminal, a fax machine, or the like. Typically, the coded overhead channels and the coded data traffic channels used by the mobile station and the base station are on the same RF carrier frequency. Advantageously, in many CDMA networks, the overhead channels and the data traffic channels are on the same RF carrier frequency in all cells (i.e., base station coverage areas) in the networks.
Each data traffic channel and overhead control channel of a base transceiver station constitutes a xe2x80x9cresourcexe2x80x9d of that base transceiver station. Ultimately, the channel resources of a base transceiver station are limited by the practical restraints imposed by the hardware in the base transceiver station. Thus, a base transceiver station is limited to servicing a certain number of data traffic channels at any one time. This limit is usually determined by the number of channel transmitter/receiver elements that are built into the BTS. If a BTS contains fifty (50) channel transceiver elements that are reserved for data traffic channels, then the BTS is limited to handling a maximum of 50 two-way communication links with 50 mobile stations. If a fifty-first mobile station tries to access the base transceiver station via its access channel, the base transceiver station will refuse the access request until one of the fifty data traffic channels becomes free.
The limitations on the number of data traffic channels served by a base transceiver station also affect the failure recovery capabilities of a base transceiver station. If a hardware or software failure occurs in a base transceiver station, an overhead channel can be lost (i.e., transmission failure) within the corresponding cell site. If the failure occurs on the Paging, Synchronization, or Access channels, the existing calls within the cell site are not lost. However, the failed base transceiver station (BTS) becomes inaccessible and no new calls can be established. If the Pilot channel is lost, not only will the BTS become inaccessible, but all existing calls are dropped within a period of only a few seconds.
To deal with the failure of an overhead channel, conventional wireless systems initiate an overhead channel switchover operation. When a switchover occurs, the BTS reconfigures an unused data traffic channel to operate as an overhead channel by using the same PN code and frequency range used by the failed overhead channel. This is an adequate solution provided that the BTS is not operating at full capacity, such that all data traffic channels are already in use. If no data traffic channel is available, the BTS drops an established call in order to free up a data traffic channel that can be reconfigured as an overhead channel. This is done even if the dropped call is a xe2x80x9c911xe2x80x9d emergency call or an important business or personal call. In any event, the dropped call is a loss for the consumer and the service provider.
To overcome the problems associated with dropping an established call in order to reconfigure an overhead channel, conventional wireless systems sometimes reserve a xe2x80x9chotxe2x80x9d standby channel element for each overhead channel. Unfortunately, this results in a loss of, for example, two to four data traffic channels, depending on how many overhead channels are used to carry the Pilot, Synchronization, Paging and Access signals.
For example, if the Pilot, Synchronization, Paging and Access signals are carried in four separate channels, four standby channels are needed and four data traffic channels are lost. If the Pilot and Paging signals are carried in separate overhead channels and the Synchronization and Access channels are combined in the same overhead channel, then three standby overhead channels ad are needed and three data traffic channels are lost.
There is therefore a need in the art for a CDMA wireless network that more efficiently utilizes the channel resources of the base transceiver stations in the network in order to serve the greatest number of mobile stations possible. There is also a need for a CDMA wireless network that suffers minimal performance degradation upon the occurrence of a failure in an overhead channel. In particular, there is a need for a CDMA wireless network that minimizes the risk of dropping an existing call in order to reconfigure a data traffic channel as an overhead channel. More particularly, there is a need for a CDMA wireless network that minimizes the risk of dropping an existing call in order to reconfigure a data traffic channel and which eliminates or reduces the need for standby channels.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a CDMA wireless network, a channel resource allocator for reallocating a data traffic channel in a soft handoff state to handle incoming calls from new mobile stations. The present invention further includes an overhead channel controller for reconfiguring an overhead channel upon an overhead channel failure in a base transceiver station (BTS). The channel resource allocator determines whether or not all data traffic channels of the BTS are in use. If all data traffic channels are in use, the channel resource allocator determines whether any of the existing calls being serviced by the data traffic channels are in a soft handoff state in which the call is connected simultaneously to two or more base transceiver stations. If so, the channel resource allocator drops the connection to the BTS in which the overhead channel failure occurred, thereby freeing up the data traffic channel. The overhead channel controller may then reconfigure the dropped data traffic channel as an overhead channel, or the channel resource allocator may reallocate the dropped data traffic channel to handle a new incoming call from a mobile station.
If more than one existing call is in a soft handoff state, the channel resource allocator drops the connection that receives the weakest reverse channel signal from the mobile station. Additionally, the channel resource allocator distinguishes between a xe2x80x9csoftxe2x80x9d handoff connection between a first BTS and a second BTS and xe2x80x9csofterxe2x80x9d handoff connection between separate sector antennas in the same BTS. The channel resource allocator drops soft handoff connections but not softer handoff connections. Furthermore, in one embodiment of the present invention, the channel resource allocator may also distinguish between an xe2x80x9cemergencyxe2x80x9d call and a xe2x80x9cnon-emergency callxe2x80x9d in determining which one(s) of two or more soft handoff calls to drop in order to reconfigure an overhead channel.
Accordingly, there is provided, in one embodiment of the present invention, for use in a wireless network comprising a plurality of base transceiver stations, each of the base transceiver stations capable of establishing and maintaining communication links with a plurality of a mobile stations by means of at least one overhead channel and a plurality of data traffic channels, an apparatus for allocating the plurality of data traffic channels comprising 1) at least one of: a) a failure detection circuit capable of detecting a failure in the at least one overhead channel of a first base transceiver station and generating a failure notification; and an access request detection circuit capable of, detecting an access request message received from an accessing one of the plurality of mobile stations and generating an access request notification; and 2) a channel allocator capable of receiving at least one of the failure notification and the access request notification and, in response thereto, terminating a first communication link between the first base transceiver station and a first selected one of the plurality of mobile stations, wherein the first selected mobile station maintains at least a second communication link with at least a second base transceiver station, and at least one of: a) reconfiguring a first data traffic channel associated with the terminated first communication link as a replacement overhead control channel replacing the failed overhead control channel and b) allocating the first data traffic channel associated with the terminated first communication link to establish a communication link with the accessing mobile station.
According to another embodiment of the present invention, the channel allocator is capable of determining if one of the plurality of data traffic channels associated with the first base transceiver station is unused prior to terminating the first communication link between the first base transceiver station and the first selected mobile station.
According to still another embodiment of the present invention, the channel allocator reconfigures an unused one of the plurality of data traffic channels associated with the first base transceiver station as the replacement overhead control channel in lieu of terminating the first communication link and reconfiguring the first data traffic channel associated with the terminated first communication link.
According to still another embodiment of the present invention, the channel allocator allocates an unused one of said plurality of data traffic channels associated with said first base transceiver station to establish a communication link with said accessing mobile station in lieu of terminating said first communication link and allocating said first data traffic channel associated with said terminated first communication link to establish a communication link with said accessing mobile station.
According to yet another embodiment of the present invention, the channel allocator further comprises a memory coupled to the overhead channel controller, wherein the memory is capable of storing status data associated with the first communication link.
According to a further embodiment of the present invention, the status data comprises a received signal strength indicator associated with the first communication link.
According to a still further embodiment of the present invention, the status data comprises handoff state data, wherein the handoff state data indicates whether the first selected mobile station associated with the first communication link maintains the at least a second communication link with the at least a second base transceiver station.
According to a yet further embodiment of the present invention, the handoff state data indicates a total number of communication links the first selected mobile station maintains with other ones of the plurality of base transceiver stations.
In yet another embodiment of the present invention, the status data further comprises a first received signal strength indicator associated with the first communication link and the overhead channel controller terminates the first communication link with the first selected mobile station if the first received signal strength indicator indicates that a signal associated with the first communication link is weaker that a second signal associated with a second communication link between the first base transceiver station and a second selected one of the plurality of mobile stations.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.